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The twisted bilayer system provides an excellent platform for the study of flatbands. In this work, we propose a bilayer hexagonal boron nitride (h-BN)-like surface plasmon crystal at a large twist angle of 38.213° due to the interlayer strong coupling, in which the adjacent pillars are in different radii. We numerically and theoretically calculate the band structure while tuning the pillar radius ratio (PRR) and the interlayer separation distance. As a result, both increasing the PRR and decreasing the separation distance contribute to the transition from weak coupling to strong coupling, leading to the flatbands with slow velocity and large density of state. Consequently, the in-layer geometry as well as the separation distance offers the degree of freedom to achieve flatbands in the bilayer surface plasmon crystal. Our work provides a fundamental understanding of the band structure of the twisted bilayer photonic system, which enriches the methods to obtain flatbands at a large twist angle.
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P2-type Na0.67Mn0.5Fe0.5O2 (MF) has attracted great interest as a promising cathode material for sodium-ion batteries (SIBs) due to its high specific capacity and low cost. However, its poor cyclic stability and rate performance hinder its practical applications, which is largely related to lattice oxygen instability. Here, we propose to coat the cathode of SIBs with Li2ZrO3, which realizes the "three-in-one" modification of Li2ZrO3 coating and Li+, Zr4+ co-doping. The synergy of Li2ZrO3 coating and Li+/Zr4+ doping improves both the cycle stability and rate performance, and the underlying modification mechanism is revealed by a series of characterization methods. The doping of Zr4+ increases the interlayer spacing of MF, reduces the diffusion barrier of Na+, and reduces the ratio of Mn3+/Mn4+, thus inhibiting the Jahn-Teller effect. The Li2ZrO3 coating layer inhibits the side reaction between the cathode and the electrolyte. The synergy of Li2ZrO3 coating and Li+, Zr4+ co-doping enhances the stability of lattice oxygen and the reversibility of anionic redox, which improves the cycle stability and rate performance. This study provides some insights into stabilizing the lattice oxygen in layered oxide cathodes for high-performance SIBs.
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In this work, we propose a multi-functional broadband terahertz polarization converter based on graphene-VO2 hybrid metamaterial, which can switch between transmissive linear-to-linear conversion and reflective linear-to-circular conversion. The function of the metamaterial can be controlled by both the temperature and the Fermi energy of the graphene. At 298K, the metamaterial converts the y-polarized wave into x-polarized wave in 0.39-1.22THz. In the meanwhile, changing the Fermi energy of graphene, the converted polarization angle can be tuned from 90° to 45°. Increasing the temperature to 358K, the incident linearly polarized wave is reflected into circularly polarized wave. On this condition, tuning the Fermi energy, the metamaterial can separately convert the linear polarization wave into left-circularly polarized wave in 1.57-2.74THz and right-circularly polarized wave in 1.13-1.59THz. Such a switchable multi-functional broadband polarization converter may achieve potential applications in compact terahertz devices and integrated terahertz circuits.
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In recent years, the integration of active materials into a metasurface to achieve tunable devices has attracted much attention. Here, we design an Au-VO2 hybrid metasurface, which can switch between quarter-wave plate and half-wave plate due to the phase transition of VO2. At 298â K, the proposed structure acts as a quarter-wave plate in the 0.87-1.2â THz band, achieving the mutual conversion between linear polarization and circular polarization. Raising the temperature to 358â K, it works as a broadband half-wave plate in the range of 0.65-1.45â THz, with the reflective chirality preservation of circular polarization and the cross-polarization conversion of linear polarization. In the above cases, the response efficiencies are both above 90%. The switchable multifunction results from the tunable geometric phase of the metasurface, where the elaborately designed Au and VO2 blocks separately bring the phase of π/2. Furthermore, the electric field and current density distributions are employed to explain the physical mechanisms leading to the different functions. Such an active broadband metasurface is expected to find applications in tunable and multifunction devices manipulating the polarization and phase of terahertz waves.
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Improving the reversibility of oxygen redox is quite significant for layered oxides cathodes in sodium-ion batteries. Herein, we for the first time simultaneously tune bulk O2 and nonbonding oxygen state for reversible oxygen redox chemistry in P2-Na0.67 Mn0.5 Fe0.5 O2 through a synergy of Li2 TiO3 coating and Li/Ti co-doping. O2- is oxidized to molecular O2 and peroxide (O2 )n- (n<2) during charging. Molecular O2 derived from transition metal (TM) migration is related to the superstructure ordering induced by Li doping. The synergy mechanism of Li2 TiO3 coating and Li/Ti co-doping on the two O-redox modes is revealed. Firstly, Li2 TiO3 coating restrains the surface O2 and inhibits O2 loss. Secondly, nonbonding Li-O-Na enhances the reversibility of O2- â(O2 )n- . Thirdly, Ti doping strengthens the TM-O bond which fixes lattice oxygen. The cationic redox reversibility is also enhanced by Li/Ti co-doping. The proposed insights into the oxygen redox reversibility are insightful for other oxide cathodes.
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In this work, we propose a multi-band terahertz perfect absorber employing the topological photonic crystal combined with VO2 and graphene. The hybrid strong coupling among the topological photonic state, the Tamm plasmon polaritons excited around the interfaces of VO2 and graphene results in the three perfect absorption bands. Benefiting from the reversible insulator-metal phase transition of VO2 and the tunable Fermi level of graphene, it can actively switch among no absorption, single-band, dual-band and multi-band absorptions around 1THz, with the absorption frequencies tunable as well. Besides, the absorption bands are sensitive to the incident angle in almost the same dispersion rate, with high absorptions in a large angle range. Moreover, the splitting frequencies between the adjacent absorption peaks strongly depend on the pair number of the alternating multilayers. Apart from the three absorption bands, there are still many absorption peaks in the large frequency range resulting from the standing waves, including other 7 peaks above 0.9 between 0.83THz and 1.55THz. Such a tunable multi-band absorber with multiple modulation methods may find extended applications in active integrated terahertz devices.
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In this paper, a metal-dielectric metamaterial absorber is proposed to achieve ultrabroadband absorption at frequencies from ultraviolet to near-infrared. Based on finite element method solutions, the average absorption of the absorber is 97.75% from 382 nm to 1100 nm, with a maximum of 99.92%, resulting from multiple resonance coupling. The influences of geometric parameters and incident conditions on absorption are investigated. Broadband and narrowband absorption changes are realized by changing incident light polarization. Polarization-independent properties can be realized by changing the dielectric structure to centrosymmetric. The average absorption of the polarization-independent structure is 97.11% from 250 nm to 1115 nm, with a maximum of 99.98%. The proposed absorber structure has wide optical applications including solar energy harvesting and light-emitting devices.
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In this paper, we design a polarization-independent and angle-insensitive broadband THz graphene metamaterial absorber based on the surface plasmon-polaritons resonance. Full-wave simulation is conducted, and the results show that the designed metamaterial absorber has an absorption above 99% in the frequency range from 1.23 THz to 1.68 THz, which refers to a very high standard. Furthermore, the absorber has the properties of tunability, and the absorption can be nearly adjusted from 1% to 99% by varying the Fermi energy level of the graphene from 0â eV to 0.7â eV. In the simulation, when the incident angles of TE and TM waves change from 0° to 60°, the average absorption keeps greater than 80%. The proposed absorber shows promising performance, which has potential applications in developing graphene-based terahertz energy harvesting and thermal emission.
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In this work, we have proposed an actively tunable bi-functional metamirror based on a bi-layer graphene structure. The metamirror acts as a spin-selective absorber under circularly polarized incidence, which behaves as nearly perfect absorption and reflection for right and left circularly polarized waves, respectively, leading to giant circular dichroism. On the other hand, it is a polarization converter under linearly polarized incidence, which reflects the linearly polarized wave into a left circularly polarized wave. Both the spin-selective absorber and the polarization converter can be actively switched between ON and OFF states, with the working frequency controlled by the voltages applied to graphene. Moreover, the metamirror is insensitive to the incidence angle, which contributes to its application as a stable single-mode spin-selective absorber and polarization converter. This bi-layer graphene structure offers a method to construct actively tunable bi-functional metamirrors, which may achieve potential applications in integrated devices, such as active spin detectors, absorbers, and quarter-wave plates for terahertz waves.
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High-voltage LiCoO2 delivers a high capacity but sharp fading is a critical issue, and the capacity decay mechanism is also poorly understood. Herein, we clarify that the escape of surface oxygen and Li-insulator Co3 O4 formation are the main causes for the capacity fading of 4.6â V LiCoO2 . We propose the inhibition of the oxygen escape for achieving stable 4.6â V LiCoO2 by tailoring the Co3d and O2p band center and enlarging their band gap with MgF2 doping. This enhances the ionicity of the Co-O bond and the redox activity of Co and improves cation migration reversibility. The inhibition of oxygen escape suppresses the formation of Li-insulator Co3 O4 and maintains the surface structure integrity. Mg acts as a pillar, providing a stable and enlarged channel for fast Li+ intercalation/extraction. The modulated LiCoO2 shows almost zero strain and achieves a record capacity retention at 4.6â V: 92 % after 100â cycles at 1C and 86.4 % after 1000â cycles at 5C.
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In this letter, we have proposed a bi-functional switchable broadband polarization converter based on the hybrid graphene-metal metasurface. Turning the bias voltage to change Fermi level Ef from 0 to 1.0 eV, the metasurface can switch between quarter-wave plate (QWP) and half-wave plate (HWP) in the frequency band 1.38-1.72 THz. Besides, the metasurface simultaneously works as a broadband QWP and HWP in different frequency range when Ef = 1.0 eV. In addition, when Ef is in the range of 0.3 eV-0.6 eV, the metasurface can work as bi-functional broadband QWP in different frequencies as well. The physical mechanism of the bi-functional polarization converter can be explained by the electric field amplitude distributions. What's more, we find that the metasurface can work well with a tolerance to the incident light polarization angle of about ± 12.5°, which can also change the converted wave from RHCP to LHCP with the incident polarization angle change of 90°. The hybrid metasurface with the advantages of switchable bi-functions, wide operating bandwidth, and ultra-thin thickness, may achieve potential applications in tunable devices for terahertz communications.
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In this Letter, we construct a graphene hybridized distributed Bragg reflector (DBR) cavity, where spatially longitudinal strong coupling occurs between the Tamm plasmon polaritons (TPPs) existing around the graphene layer and the cavity mode (CM) existing in the DBR cavity. As a result, two hybrid polariton modes emerge, which contain both the TPP and the CM components. In the simulation, we demonstrate that the resonant frequencies and the damping rates of the polariton modes can be actively tuned by the graphene Fermi level and the incident angle of light. Besides, the coupling strength and the damping rates are also passively tuned by the pair number of the layers in the DBR. Theoretically, we analyze the TPP-CM strong coupling by the coupled harmonic oscillator equations, which help to explain the regulation process. The controllable TPP-CM longitudinal strong coupling with two absorption bands may achieve potential applications in developing graphene-based active optoelectronic and polaritonic devices in terahertz waves.
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Li-rich Mn-based (LRMO) cathode materials have attracted widespread attention due to their high specific capacity, energy density, and cost-effectiveness. However, challenges such as poor cycling stability, voltage deca,y and oxygen escape limit their commercial application in liquid Li-ion batteries. Consequently, there is a growing interest in the development of safe and resilient all-solid-state batteries (ASSBs), driven by their remarkable safety features and superior energy density. ASSBs based on LRMO cathodes offer distinct advantages over conventional liquid Li-ion batteries, including long-term cycle stability, thermal and wider electrochemical windows stability, as well as the prevention of transition metal dissolution. This review aims to recapitulate the challenges and fundamental understanding associated with the application of LRMO cathodes in ASSBs. Additionally, it proposes the mechanisms of interfacial mechanical and chemical instability, introduces noteworthy strategies to enhance oxygen redox reversibility, enhances high-voltage interfacial stability, and optimizes Li+ transfer kinetics. Furthermore, it suggests potential research approaches to facilitate the large-scale implementation of LRMO cathodes in ASSBs.
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Swift heavy ion (SHI) irradiation is an effective method for modulating the properties of thin oxide films by introducing defects, strains, and structural transformations. Here, we applied 516 MeV Xe31+ irradiation to BaTiO3 (BTO) thin films grown on Nb:SrTiO3 substrates to induce the generation of tracks and nanohillocks. Memristors with BTO films irradiated at a fluence of 5 × 1010 ions cm-2 displayed excellent retention and endurance characteristics. Moreover, the memristors exhibited highly stable synaptic plasticity functions such as excitatory/inhibitory post-synaptic currents (E/IPSC) and paired-pulse facilitation/depression (PPF/D). The memristors achieved a discrimination accuracy of 92.5% on given handwritten digit data by an artificial neural network with supervised learning. These results verify that the judicious application of SHI irradiation on thin oxide films is a viable strategy for exploring neuromorphic computation.
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Alkali ion (Li, Na, and K) batteries as a new generation of energy storage devices are widely applied in portable electronic devices and large-scale energy storage equipment. The recent focus has been devoted to develop universal anodes for these alkali ion batteries with superior performance. Transition metal sulfides can accommodate alkaline ions with large radius to travel freely between layers due to its large interlayer spacing. Moreover, the composite with carbon material can further improve electrical conductivity of transition metal sulfides and reduce the electron transfer resistance, which is beneficial for the transport of alkali ions. Herein, we designed zeolitic imidazolate framework (ZIF)-derived hollow structures CoS/C for excellent alkali ion (Li, Na, and K) battery anodes. The porous carbon framework can improve the conductivity and effectively buffer the stress-induced structural damage. The ZIF-derived CoS/C anodes maintain a reversible capacity of 648.9, and 373.2, 224.8 mAh g-1 for Li, Na, and K ion batteries after 100 cycles, respectively. Its outstanding electrochemical performance is considered as a universal anode material for Li, Na, and K ion batteries.
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In this paper, we theoretically demonstrate a dual-band independently tunable absorber consisting of a stacked graphene nanodisk and graphene layer with nanohole structure, and a metal reflector spaced by insulator layers. This structure exhibits a dipole resonance mode in graphene nanodisks and a quadrupole resonance mode in the graphene layer with nanoholes, which results in the enhancement of absorption over a wide range of incident angles for both TE and TM polarizations. The peak absorption wavelength is analyzed in detail for different geometrical parameters and the Fermi energy levels of graphene. The results show that both peaks of the absorber can be tuned dynamically and simultaneously by varying the Fermi energy level of graphene nanodisks and graphene layer with nanoholes structure. In addition, one can also independently tune each resonant frequency by only changing the Fermi energy level of one graphene layer. Such a device could be used as a chemical sensor, detector or multi-band absorber.
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By introducing the scattering probability of a subsurface defect (SSD) and statistical distribution functions of SSD radius, refractive index, and position, we derive an extended bidirectional reflectance distribution function (BRDF) from the Jones scattering matrix. This function is applicable to the calculation for comparison with measurement of polarized light-scattering resulting from a SSD. A numerical calculation of the extended BRDF for the case of p-polarized incident light was performed by means of the Monte Carlo method. Our numerical results indicate that the extended BRDF strongly depends on the light incidence angle, the light scattering angle, and the out-of-plane azimuth angle. We observe a 180 degrees symmetry with respect to the azimuth angle. We further investigate the influence of the SSD density, the substrate refractive index, and the statistical distributions of the SSD radius and refractive index on the extended BRDF. For transparent substrates, we also find the dependence of the extended BRDF on the SSD positions.